Friction stir welding or joining is a method for interconnecting members to be joined without fusion (see, for example, Patent Literature 1).
In the method, a workpiece comprising stacked members to be joined together is rested on a support tool or backing member. While rotated, a joining tool is pushed against the workpiece so that the materials softened due to frictional heat and plastic flow are assimilated together through stirring.
Then, the joining tool is released from the workpiece to allow the assimilated materials to solidify, thereby joining the members together.
The joining tool comprises a cylindrical shoulder and a short cylindrical pin which is coaxially contiguous with the shoulder, is protruded as a tip of the tool and is smaller in outer diameter than the shoulder.    [Patent Literature 1] JP 2004-136365A